Coagulase-negative staphylococci (CNS) are the most frequently isolated group of microorganisms from the bovine udder. Research has shown that certain CNS species can be protective, while other species are prone to decrease milk quality and production. Our objective was to describe the distribution of CNS species for the lactation period and parities in Canadian herds, as an initial effort to understand the disease. Cows were sampled randomly across Canada in 2007 and 2008. In total, 1965 cows had udder infection with CNS, being compared with a total of 6067 cows sampled in the study. Chi-squared was used to test different prevalences (P < 0.05). The 5 most common CNS species identified in were S. chromogenes (49%), S. simulans (17%), S. xylosus (11%), S. haemolyticus (7%) and S. epidermidis (4%). Parity and lactation prevalences are presented in Table 1. S. chromogenes was predominant in heifers, and its prevalence increased during the lactation. S. epidermidis was mostly isolated from older cows and the prevalence did not increase as lactation progressed. S. haemolyticus isolated equally often in all parities; its prevalence increased during lactation. S. simulans seems to be more isolated in heifers, and its prevalence did not increase over the lactation. S. xylosus fluctuates over parities and its prevalence increased during the lactation. The distribution of these CNS species differed over parities and lactation period. The prevalence of species such as S. chromogenes, which maybe less pathogenic, was very high in heifers compared with older cows. Throughout lactation, it seems that most species can persist or reinfect udder. Understanding the distribution of CNS species will possibly lead to selective management practices and may lead to identification of species or genotypes that act as protective udder pathogens. Table 1. Relative frequency (%) of CNS species distribution over parity and over lactation thirds

CNS species	Parity					Lactation thirds
1	2	3	≥4		1	2	3	>305 d
S. chromogenes	60	23	24	26		8	8	11	11
S. epidermidis	1	3	3	5		1	1	0.5	1
S. haemolyticus	5	5	6	5		1	1.2	2	2
S. simulans	15	9	10	13		4	2	2	3
S. xylosus	6	9	7	10		1	1	2	3